1. Field of the Invention
The present invention relates to a fluorine adsorbent/desorbent applicable repeatedly for adsorbing and desorbing of fluorine contained in a zinc electrolytic solution for electro-refining, the fluorine being contained by dissolving a zinc containing material containing the fluorine, as well as a method for removing the fluorine using this fluorine adsorbent/desorbent. (The term “fluorine adsorbent/desorbent” used in the specification refers to chemicals that can adsorb and desorb (in other words, attach and detach) fluorine in a given solution as described above.)
2. Description of the Related Art
In conventional zinc electro-refining, electrolytically refined zinc of purity 99.99% is manufactured through, desulfurization of zinc sulfide concentrate as raw material by roasting, dissolving and leaching out of the roasted zinc concentrate into acid solution such as a sulfuric acid solution to prepare a zinc electrolytic solution, and then carrying out electrolysis.
On the other hand, the following method is used in a steel mill (or steel recycle plant) in recent years: a zinc containing material (simply referred to as a “zinc containing material” containing at least zinc and fluorine) is first recovered from steel dust emitted during a steel production process, or zinc residues resulting from a zinc plating process. Then, the zinc containing material is mixed with roasted concentrate. The mixture is then dissolved into an acid solution to obtain a zinc electrolytic solution. The solution is then served for electrolysis to obtain zinc. This method may reduce the costs of the materials.
This zinc containing material is not a sulfide, so it needs no roasting. This allows a reduction in manufacturing costs for an intermediate process. However, the zinc containing material may cause problems because it has halogen elements. The halogen element is not limited to fluorine. However, the present application focuses on fluorine, which causes problems in zinc electro-refining as described below in detail.
That is, if the contained fluorine is come into an electrolytic solution, during zinc electro-deposition, an aluminum cathode may be corroded by fluorine to prevent electro deposited zinc from being stripped. This precludes the aluminum plate to be repeatedly used, thus significantly hindering mass productivity.
Thus, several methods have been proposed.
First, Patent Document 1 discloses three methods for removing the contained fluorine from the electrolytic solution: a method of washing a zinc source (raw material of zinc) (this method will be referred to as a conventional method 1 below), a method of removing fluorine by sulfating roasting or the like (this method will be referred to as a conventional method 2 below), and a method of electro-deposition of zinc in two steps; in the first step, a sulfuric acid zinc solution free from fluorine ions is used to deposit predetermined zinc, and in the second step, a sulfuric acid zinc sulfate solution containing fluorine ions is used to obtain zinc (this method will be referred to as a conventional method 3 below).
Further, Patent Document 2 discloses a method for removing fluorine directly from a zinc electrolytic solution using cerium hydroxide (this method will be referred to as a conventional method 4).
Moreover, Patent Document 3 discloses an apparatus that removes fluorine and the like for waste water treatment. Further, Patent Document 4 discloses an anion adsorbent consist of amorphous iron hydroxide group precipitate for anions such as fluorine ion for waste water treatment.    [Patent Document 1] Japanese Patent Laid-Open No. 1992-221089    [Patent Document 2] Japanese Patent Laid-Open No. 2002-105685    [Patent Document 3] Japanese Patent Laid-Open No. 2004-351392    [Patent Document 4] Japanese Patent Laid-Open No. 2003-334542
However, the conventional method 1 may not sufficiently remove fluorine.
Further, the conventional method 2 may increase costs and is thus economically disadvantageous.
Furthermore, the conventional method 3 requires the process to be divided into two steps. Accordingly, it can eliminate the effects of fluorine but it has the drawbacks in increasing of the costs for investment of refining plants and equipment with complicated process management.
Moreover, the conventional method 4 is economically disadvantageous because the price of cerium hydroxide used for adsorbing fluorine is expensive. The conventional method 4 is expected to be difficult to actually apply on a plant scale.
Thus, the conventional methods 1 to 4 disclosed in Patent Documents 1 and 2 may increase total cost even though they successfully reduce material costs. As a result, these methods may loose their industrial advantage of enabling resource recycling while reducing costs by using a zinc containing material obtained by recovering steel dust and the like, which have been conventionally wasted.
Patent Document 3 discloses a technique for removing fluorine and the like which is applied only to an industrial waste water treatment (waste solution treatment). Specifically, Patent Document 3 discloses a method of adding calcium to fluorine as a water-soluble metal compound to deposit calcium fluoride and then removing fluorine and the like. That is, this is different from the method for removing fluorine according to the present invention in the order of treatment and the configuration of treatment steps.
Further, Patent Document 4 discloses a technique only applied to industrial waste water treatment (waste solution treatment) and relating mainly to a method for removing heavy metals. In particular, as is apparent from the embodiments in Patent Document 4, the method disclosed in Patent Document 4 relate to removal of heavy metal such as arsenic, antimony, selenium, chromium, molybdenum or cadmium from waste water mainly.
However, in general, when the concentration of fluorine in a zinc electrolytic solution exceeds 20 mg/L, an aluminum cathode is remarkably corroded in zinc electro-refining. Consequently, tendency that the deposited zinc cannot be stripped off occurs. This results the difficulty in performing an electro-refining operation and an extreme reduction in the lifetime of the aluminum plate for repeating use. The manufacturing costs may thus increase. Therefore, it has been desirable to maintain the concentration of fluorine in the zinc electrolytic solution below 20 mg/L.
As apparent from the above description, a technique for removing fluorine from a zinc electrolytic solution (technique for absorption and desorption of fluorine) has been desired. The technique allows using a zinc containing material containing fluorine without preventing a cost reduction achieved by the use of an inexpensive material, that is, zinc residues, from causing total costs to increase during the subsequent steps. Also the technique is capable of reducing facility costs and ease process management. The present inventors have completed the present invention according to this technique.